Pressure sensitive adhesives have for a long time been used for attaching medical devices, such as ostomy appliances, dressings (including wound dressings), wound drainage bandages, devices for collecting urine, orthoses and prostheses to the skin.
It has been reported that humans for short periods can sweat more than 20,000 g/m2/24 h, (Main, K., K. O. Nilsson, and N. E. Skakkebaek, 1991, Influence of sex and growth hormone deficiency on sweating, Scand. J. Clin. Lab Invest 51:475-480).
Thus, the moisture handling ability of skin contact adhesives, i.e. both the water absorption capacity and the moisture vapour transmission rate of the adhesive, is important.
When designing a skin adhesive one of the major issues is to keep the skin relatively dry underneath the adhesive to prevent maceration. Maceration occurs when skin is unable to get rid of moisture from transpiration and results in degradation of the skin barrier function.
Usually, skin adhesive keeps the skin dry by being water permeable. This allows moisture to transport through the adhesive from the skin side to the outer side, where it is allowed to evaporate. This mechanism is not usable for ostomy skin adhesives because a water impermeable layer covers the outer side of an ostomy adhesive. The water impermeable layer prevents ostomy discharge to enter the adhesive from the outside. Thus, evaporation of moisture is not possible. Hence, adhesive compositions used for ostomy appliances are made water absorbent. Absorbing particles or hydrocolloids (HC) are mixed into a hydrophobic adhesive matrix to absorb moisture from the skin and thereby keeping the skin relatively dry. This technique is well known in the art (see for example U.S. Pat. No. 6,451,883) and forms the basis for all commercially available ostomy adhesives.
The adhesive matrix in traditional, state of the art of ostomy adhesives without HC particles is very hydrophobic with very low water permeability. The only way water can transmit within the adhesive is through the hydrocolloid particles that are mixed therein. As these particles are much smaller than the total thickness of the adhesive layer, the only way water can migrate into the adhesive is if the HC particles touch each other and form bridges for water to permeate through. This limit in water transportation dictates a relatively high loading of hydrocolloid particles in the adhesive such that enough particles touch each other. As the particles used are hard relative to the adhesive matrix, this addition of a large quantity of particles makes the adhesive hard and uncomfortable for the user.
By using a water permeable adhesive matrix such as the adhesive described in WO 05/032401 the water transportation is less dependent on the number of particles that touches each other, fewer particles are needed to ensure proper water mobility in the adhesive.
Unfortunately, by reducing the amount of absorbing particles, the absorption capacity and rate is reduced. Water absorption in the adhesive is driven by a difference in vapour pressure between the skin and the inside of the adhesive. The vapour pressure over hydrocolloids grows rapidly towards the equilibrium vapour pressure of water as water is absorbed by the particles. As vapour pressure grows in the hydrocolloids, the driving force reduces and water transport gets slower. This would also be the case for a regular HC adhesive with impermeable adhesive matrix, but here, water transport is helped by the expanding particles starting to touch more neighbour particles and form more bridges. This way the resistance against water flux reduces and compensates for the lower driving force.
Thus, reducing the amount of particles not only reduces the absorption capacity but also reduces the absorption rate of the adhesive. Using the known technology today, it is not possible to make an ostomy adhesive that is soft, have high water absorption capacity and also have a relatively high and constant transient water uptake.
The present invention provides an absorbing adhesive with very low particle loading and still having high water absorption capacity.